Industrial and power generation gas turbines have control systems (“controllers”) that monitor and control their operation. These controllers govern the combustion system of the gas turbine. Dry Low NOx (DLN) combustion systems are designed to minimize emissions of NOx from gas turbines. The controller executes an algorithm to ensure safe and efficient operation of the DLN combustion system. Conventional DLN algorithms receive as inputs measurements of the actual exhaust temperature of the turbine and the actual operating compressor pressure ratio. DLN combustion systems typically rely on the measured turbine exhaust temperature and compressor pressure ratio to set the gas turbine operating condition.
Conventional scheduling algorithms for DLN combustion systems do not generally take into account variations in compressor inlet pressure loss, turbine backpressure, compressor inlet humidity, low pressure turbine speed, high pressure turbine speed, and bypass valve air split. Conventional scheduling algorithms generally assume that ambient conditions, e.g., compressor inlet humidity, compressor inlet pressure loss, and turbine back pressure remain at certain defined constant conditions or that variations in these conditions do not significantly affect the target combustor firing temperature.
Compressor inlet pressure loss and turbine backpressure levels will vary from those used to define the DLN combustion settings. The NOx emissions and CO emissions from the gas turbine may increase beyond prescribed limits, if the conventional DLN combustion system is not adjusted as environmental conditions change. Seasonal variations in humidity or changes in turbine inlet humidity from various inlet conditioning devices, for example, evaporative cooler, fogging systems, can influence the operation of a DLN combustion system. As the ambient conditions change with the seasons, the settings of DLN combustion systems are often manually adjusted to account for ambient seasonal variations.
The Dry Low NOx (DLN) combustion system was modified for application on a two-shaft, compressor drive, single can, combustion gas turbine. The program required that the combustion system meet both CO and NOx emissions requirements at 50% turndown operation. A combustion bypass valve was designed into the DLN system to change the fuel to air ratio at the head end and thus flame temperature to meet the CO requirements at low loads. In the prior art, there existed no way to schedule the bypass valve air split to meet the CO emissions requirements.
A corrected parameter control approach was to be used to control the turbine operation. Exhaust temperature target adjustments were to be made based on specific humidity, compressor inlet pressure and compressor exhaust pressure. A two-shaft system added several more variables to the development of the exhaust temperature correction since its shaft speeds are not fixed. The addition of high pressure and low pressure turbine speeds, as well as second stage nozzle guide vanes and combustion bypass air increased the number of inputs into the algorithm and complicated the control. While turbine exhaust could be used to control high pressure turbine speed, low pressure turbine speed, and nozzle guide vanes, there existed no way to control the bypass air split.